1. Field of the Invention
The present invention relates to high power bus assemblies for converters with high speed switches that are modulated to convert electrical power from AC to DC, or from DC to AC.
2. Description of Related Art
There are many instances in which conversion bet ween AC electrical energy and DC electrical energy can be useful. For example, U.S. Pat. No. 5,083,039 to Richardson, et al., describes a variable speed wind turbine that utilizes a converter to convert the generated variable frequency AC power to DC power. A second converter then converts this DC power back to AC power, but at a fixed frequency that is compatible with the power grid. The converters described therein are switching converters controlled by pulse width modulation techniques to provide a current-controlled waveform or a voltage-controlled waveform if desired.
A switching converter includes a plurality of high speed switches that are quickly modulated to produce a predetermined waveform. Higher switching speeds allow greater control of the predetermined waveform. One limit upon the switching speed of a converter is the impedance of the input and output buses. An inductive impedance substantially attenuates the high frequency response of a bus, slowing effective switching speed, reducing conversion efficiency, and lessening waveform control so that the output waveform is less accurate. Another impedance-related problem is the appearance of voltage transients which can harm semiconductor devices. Voltage transients during high speed switching are caused by inductive impedances in the bus. Therefore, the voltage transients can be lowered by reducing the characteristic impedance of the bus.
The design of a bus in large part determines its impedance. To reduce impedance at high (microwave) frequencies, parallel plate transmission lines have been used extensively. However, outside of high frequency uses, parallel plate transmission lines have not been widely used. The copending patent application, Ser. No. 07/728,112, "Low Impedance Bus for Power Electronics" referenced above discloses use of a parallel plate transmission line in a power electronics environment. The bus configuration disclosed therein is designed to balance current flows for low impedance and low noise. Despite the advantages of that bus configuration, its overall package is costly in terms of space.
For low power electrical circuits, space and cost savings have been realized by using laminated circuits that have a stack of conductive layers alternating with dielectric layers. Such low power laminated circuits are usually manufactured with "prepreg" dielectric layers that, under heat and pressure, conform about the thin conductors. However, for high power applications, laminated circuits are not used. One explanation is difficulty in manufacturing. High power conductors have a large conductive cross-section, which require that the conductors have much greater thicknesses. A conventional prepreg process is not practical for manufacturing with these large thicknesses because the prepreg layer cannot provide enough dielectric to fill in the gaps. Another problem with high power laminated circuits is arcing between closely positioned high voltage conductors. Arcing is a problem when connecting conductors that are in different layers, and particularly when the connection passes through another conductive layer.
It would be an advantage to supply a high power bus with low impedance in a compact, robust package that is mechanically strong and can safely conduct large current at high voltages. Such a bus would allow reliable converter operation with efficiency and accuracy.